Day 15: Warehouse Woes

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FAQ

  •  gentooer   ( @gentooer@programming.dev ) 
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    8 hours ago

    Haskell

    Runs in 12 ms. I was very happy with my code for part 1, but will sadly have to rewrite it completely for part 2.

    Code
    import Control.Monad.State.Lazy
    import qualified Data.Map.Strict as M
    
    type Coord = (Int, Int)
    data Block = Box | Wall
    type Grid = M.Map Coord Block
    
    parse :: String -> ((Coord, Grid), [Coord])
    parse s =
        let robot = head
                [ (r, c)
                | (r, row) <- zip [0 ..] $ lines s
                , (c, '@') <- zip [0 ..] row
                ]
            grid = M.fromAscList
                [ ((r, c), val)
                | (r, row) <- zip [0 ..] $ lines s
                , (c, Just val) <- zip [0 ..] $ map f row
                ]
        in  ((robot, grid), go s)
        where
            f 'O' = Just Box
            f '#' = Just Wall
            f _ = Nothing
            go ('^' : rest) = (-1,  0) : go rest
            go ('v' : rest) = ( 1,  0) : go rest
            go ('<' : rest) = ( 0, -1) : go rest
            go ('>' : rest) = ( 0,  1) : go rest
            go (_   : rest) =            go rest
            go [] = []
    
    add :: Coord -> Coord -> Coord
    add (r0, c0) (r1, c1) = (r0 + r1, c0 + c1)
    
    moveBoxes :: Coord -> Coord -> Grid -> Maybe Grid
    moveBoxes dr r grid = case grid M.!? r of
        Nothing   -> Just grid
        Just Wall -> Nothing
        Just Box  ->
            M.insert (add r dr) Box . M.delete r <$> moveBoxes dr (add r dr) grid
    
    move :: Coord -> State (Coord, Grid) Bool
    move dr = state $ \(r, g) -> case moveBoxes dr (add r dr) g of
        Just g' -> (True, (add r dr, g'))
        Nothing -> (False, (r, g))
    
    moves :: [Coord] -> State (Coord, Grid) ()
    moves = mapM_ move
    
    main :: IO ()
    main = do
        ((robot, grid), movements) <- parse <$> getContents
        let (_, grid') = execState (moves movements) (robot, grid)
        print $ sum [100 * r + c | ((r, c), Box) <- M.toList grid']
    
  •  ystael   ( @ystael@beehaw.org ) 
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    7 hours ago

    J

    Nothing much to say about today’s. I think I wrote basically the same code you’d write in Python, just with fewer characters, more of which are punctuation. I did learn a little bit more about how to use J’s step debugger, and that / is specifically a right fold, so you can use it on a dyad with arguments of different types as long as the list argument is the left one.

    data_file_name =: '15.data'
    lines =: cutopen fread data_file_name
    NB. instructions start with the first line not containing a # character
    start_of_moves =: 0 i.~ '#' e."1 > lines
    grid =: ,. > start_of_moves {. lines
    start_row =: 1 i.~ '@' e."1 grid
    start_col =: '@' i.~ start_row { grid
    pos =: start_row, start_col
    grid =: '.' ( start_of_moves }. lines
    translate_move =: monad define"0
       if. y = '>' do. 0 1
       elseif. y = '^' do. _1 0
       elseif. y = '<' do. 0 _1
       elseif. y = 'v' do. 1 0
       else. 0 0 end.
    )
    moves =: translate_move move_instructions
    NB. pos step move updates grid as needed and returns the new position
    step =: dyad define"1 1
       new_pos =. x + y
       if. '#' = (< new_pos) { grid do. x  NB. obstructed by wall
       elseif. '.' = (< new_pos) { grid do. new_pos  NB. free to move
       else.  NB. it's 'O', need to push a stack
          p =. new_pos  NB. pointer to box at end of stack
          while. 'O' = (< p) { grid do. p =. p + y end.
          if. '#' = (< p) { grid do. x  NB. stack is blocked
          else.  NB. move stack
             grid =: 'O.' (< p ,: new_pos)} grid
             new_pos
          end.
       end.
    )
    score =: dyad define"0 2
       +/ ; ((<"0) 100 * i.#y) +&.> (< @: I. @: = & x)"1 y
    )
    final_pos =: step~/ |. pos , moves  NB. / is a right fold
    result1 =: 'O' score grid
    
    translate_cell =: monad define"0
       if. y = '#' do. '##'
       elseif. y = '.' do. '..'
       elseif. y = 'O' do. '[]'
       else. '@.' end.
    )
    grid2 =: (,/ @: translate_cell)"1 ,. > start_of_moves {. lines
    start_row2 =: 1 i.~ '@' e."1 grid2
    start_col2 =: '@' i.~ start_row { grid2
    pos =: start_row2, start_col2
    grid2 =: '.' (< pos)} grid2  NB. erase the @
    NB. (grid; box_pos) try_push dir attempts to push the box at box_pos one
    NB. cell in direction dir. box_pos can be either the left or right cell of
    NB. the box. it returns (grid; success) where grid is the maybe-changed grid
    NB. and success is whether the box moved. if any box that would be pushed
    NB. cannot move, this box cannot move either and the grid does not change.
    try_push =: dyad define"1 1
       'grid pos' =. x
       if. ']' = (< pos) { grid do. pos =. pos + 0 _1 end.  NB. make pos left cell
       source_cells =. pos ,: pos + 0 1
       if. 0 = {: y do.  NB. moving up or down
          target_cells =. (pos + y) ,: (pos + y + 0 1)  NB. cells we move into
       elseif. y -: 0 _1 do. target_cells =. 1 2 $ pos + y  NB. moving left
       else. target_cells =. 1 2 $ pos + y + 0 1 end.  NB. moving right
       NB. Have to check target cells one at a time because pushing a box up or
       NB. down may vacate the other target cell, or it may not
       trial_grid =. grid
       for_tc. target_cells do.
          NB. if a target cell is blocked by wall, fail
          if. '#' = (< tc) { trial_grid do. grid; 0 return.
          elseif. '[]' e.~ (< tc) { trial_grid do.
             'trial_grid success' =. (trial_grid; tc) try_push y
             if. -. success do. grid; 0 return. end.
          end.
       end.
       NB. at this point either target_cells are clear or we have returned failure,
       NB. so push the box
       grid =. '[]' (<"1 source_cells +"1 y)} '.' (<"1 source_cells)} trial_grid
       grid; 1
    )
    NB. (grid; pos) step2 move executes the move and returns new (grid; pos)
    step2 =: dyad define"1 1
       'grid pos' =. x
       new_pos =. pos + y
       if. '#' = (< new_pos) { grid do. grid; pos  NB. obstructed by wall
       elseif. '.' = (< new_pos) { grid do. grid; new_pos  NB. free to move
       else.  NB. need to push a box
          'new_grid success' =. (grid; new_pos) try_push y
          if. success do. new_grid; new_pos else. grid; pos end.
       end.
    )
    'final_grid final_pos' =: > (step2~ &.>)/ (<"1 |. moves) , <(grid2; pos)
    result2 =: '[' score final_grid
    
  • Rust

    Part 2 was a bit tricky. Moving into a box horizontally works mostly the same as for part 1, for the vertical case I used two recursive functions. The first recurses from the left and right side for each box just to find out if the entire tree can be moved. The second function actually does the moving in a similar recursive structure, but now with the knowledge that all subtrees can actually be moved.

    Lots of moving parts, but at least it could very nicely be debugged by printing out the map from the two minimal examples after each round.

    Solution
    use euclid::{default::*, vec2};
    
    // Common type for both parts. In part 1 all boxes are BoxL.
    #[derive(Clone, Copy)]
    enum Spot {
        Empty,
        BoxL,
        BoxR,
        Wall,
    }
    
    impl From for Spot {
        fn from(value: u8) -> Self {
            match value {
                b'.' | b'@' => Spot::Empty,
                b'O' => Spot::BoxL,
                b'#' => Spot::Wall,
                other => panic!("Invalid spot: {other}"),
            }
        }
    }
    
    fn parse(input: &str) -> (Vec>, Point2D, Vec>) {
        let (field_s, moves_s) = input.split_once("\n\n").unwrap();
        let mut field = Vec::new();
        let mut robot = None;
        for (y, l) in field_s.lines().enumerate() {
            let mut row = Vec::new();
            for (x, b) in l.bytes().enumerate() {
                row.push(Spot::from(b));
                if b == b'@' {
                    robot = Some(Point2D::new(x, y).to_i32())
                }
            }
            field.push(row);
        }
    
        let moves = moves_s
            .bytes()
            .filter(|b| *b != b'\n')
            .map(|b| match b {
                b'^' => vec2(0, -1),
                b'>' => vec2(1, 0),
                b'v' => vec2(0, 1),
                b'<' => vec2(-1, 0),
                other => panic!("Invalid move: {other}"),
            })
            .collect();
        (field, robot.unwrap(), moves)
    }
    
    fn gps(field: &[Vec]) -> u32 {
        let mut sum = 0;
        for (y, row) in field.iter().enumerate() {
            for (x, s) in row.iter().enumerate() {
                if let Spot::BoxL = s {
                    sum += x + 100 * y;
                }
            }
        }
        sum as u32
    }
    
    fn part1(input: String) {
        let (mut field, mut robot, moves) = parse(&input);
        for m in moves {
            let next = robot + m;
            match field[next.y as usize][next.x as usize] {
                Spot::Empty => robot = next, // Move into space
                Spot::BoxL => {
                    let mut search = next + m;
                    let can_move = loop {
                        match field[search.y as usize][search.x as usize] {
                            Spot::BoxL => {}
                            Spot::Wall => break false,
                            Spot::Empty => break true,
                            Spot::BoxR => unreachable!(),
                        }
                        search += m;
                    };
                    if can_move {
                        robot = next;
                        field[next.y as usize][next.x as usize] = Spot::Empty;
                        field[search.y as usize][search.x as usize] = Spot::BoxL;
                    }
                }
                Spot::Wall => {} // Cannot move
                Spot::BoxR => unreachable!(),
            }
        }
        println!("{}", gps(&field));
    }
    
    // Transform part 1 field to wider part 2 field
    fn widen(field: &[Vec]) -> Vec> {
        field
            .iter()
            .map(|row| {
                row.iter()
                    .flat_map(|s| match s {
                        Spot::Empty => [Spot::Empty; 2],
                        Spot::Wall => [Spot::Wall; 2],
                        Spot::BoxL => [Spot::BoxL, Spot::BoxR],
                        Spot::BoxR => unreachable!(),
                    })
                    .collect()
            })
            .collect()
    }
    
    // Recursively find out whether or not the robot can move in direction `dir` from `start`.
    fn can_move_rec(field: &[Vec], start: Point2D, dir: Vector2D) -> bool {
        let next = start + dir;
        match field[next.y as usize][next.x as usize] {
            Spot::Empty => true,
            Spot::BoxL => can_move_rec(field, next, dir) && can_move_rec(field, next + vec2(1, 0), dir),
            Spot::BoxR => can_move_rec(field, next - vec2(1, 0), dir) && can_move_rec(field, next, dir),
            Spot::Wall => false,
        }
    }
    
    // Recursively execute a move for vertical directions into boxes.
    fn do_move(field: &mut [Vec], start: Point2D, dir: Vector2D) {
        let next = start + dir;
        match field[next.y as usize][next.x as usize] {
            Spot::Empty | Spot::Wall => {}
            Spot::BoxL => {
                do_move(field, next, dir);
                do_move(field, next + vec2(1, 0), dir);
                let move_to = next + dir;
                field[next.y as usize][next.x as usize] = Spot::Empty;
                field[next.y as usize][next.x as usize + 1] = Spot::Empty;
                field[move_to.y as usize][move_to.x as usize] = Spot::BoxL;
                field[move_to.y as usize][move_to.x as usize + 1] = Spot::BoxR;
            }
            Spot::BoxR => {
                do_move(field, next - vec2(1, 0), dir);
                do_move(field, next, dir);
                let move_to = next + dir;
                field[next.y as usize][next.x as usize - 1] = Spot::Empty;
                field[next.y as usize][next.x as usize] = Spot::Empty;
                field[move_to.y as usize][move_to.x as usize - 1] = Spot::BoxL;
                field[move_to.y as usize][move_to.x as usize] = Spot::BoxR;
            }
        }
    }
    
    fn part2(input: String) {
        let (field1, robot1, moves) = parse(&input);
        let mut field = widen(&field1);
        let mut robot = Point2D::new(robot1.x * 2, robot1.y);
        for m in moves {
            let next = robot + m;
            match field[next.y as usize][next.x as usize] {
                Spot::Empty => robot = next, // Move into space
                Spot::BoxL | Spot::BoxR if m.y == 0 => {
                    let mut search = next + m;
                    let can_move = loop {
                        match field[search.y as usize][search.x as usize] {
                            Spot::BoxL | Spot::BoxR => {}
                            Spot::Wall => break false,
                            Spot::Empty => break true,
                        }
                        search += m;
                    };
                    if can_move {
                        robot = next;
                        // Shift boxes by array remove/insert
                        field[next.y as usize].remove(search.x as usize);
                        field[next.y as usize].insert(next.x as usize, Spot::Empty);
                    }
                }
                Spot::BoxL | Spot::BoxR => {
                    if can_move_rec(&field, robot, m) {
                        do_move(&mut field, robot, m);
                        robot = next;
                    }
                }
                Spot::Wall => {} // Cannot move
            }
        }
        println!("{}", gps(&field));
    }
    
    util::aoc_main!();
    

    Also on github

  • TypeScript

    Not very optimized code today. Basically just a recursive function

    Code
    import fs from "fs";
    
    type Point = {x: number, y: number};
    
    enum Direction {
        UP = '^',
        DOWN = 'v',
        LEFT = '<',
        RIGHT = '>'
    }
    
    const input = fs.readFileSync("./15/input.txt", "utf-8").split(/[\r\n]{4,}/);
    const warehouse: string[][] = input[0]
        .split(/[\r\n]+/)
        .map(row => row.split(""));
    const movements: Direction[] = input[1]
        .split("")
        .map(char => char.trim())
        .filter(Boolean)
        .map(char => char as Direction);
    
    // Part 1
    console.info("Part 1: " + solve(warehouse, movements));
    
    // Part 2
    const secondWarehouse = warehouse.map(row => {
        const newRow: string[] = [];
        for (const char of row) {
            if (char === '#') { newRow.push('#', '#'); }
            else if (char === 'O') { newRow.push('[', ']'); }
            else if (char === '.') { newRow.push('.', '.'); }
            else { newRow.push('@', '.'); }
        }
        return newRow;
    });
    console.info("Part 2: " + solve(secondWarehouse, movements));
    
    function solve(warehouse: string[][], movements: Direction[]): number {
        let _warehouse = warehouse.map(row => [...row]); // Take a copy to avoid modifying the original
        const robotLocation: Point = findStartLocation(_warehouse);
    
        for (const move of movements) {
            // Under some very specific circumstances in part 2, tryMove returns false, but the grid has already been modified
            // "Fix" the issue ba taking a copy so we can easily revert all changes made
            // Slow AF of course but rest of this code isn't optimized either, so...
            const copy = _warehouse.map(row => [...row]);
        
            if (tryMove(robotLocation, move, _warehouse)) {
                if (move === Direction.UP) { robotLocation.y--; }
                else if (move === Direction.DOWN) { robotLocation.y++; }
                else if (move === Direction.LEFT) { robotLocation.x--; }
                else { robotLocation.x++; }
            } else {
                _warehouse = copy; // Revert changes
            }
        }
    
        // GPS
        let result = 0;
        for (let y = 0; y < _warehouse.length; y++) {
            for (let x = 0; x < _warehouse[y].length; x++) {
                if (_warehouse[y][x] === "O" || _warehouse[y][x] === "[") {
                    result += 100 * y + x;
                }
            }
        }
        return result;
    }
    
    function tryMove(from: Point, direction: Direction, warehouse: string[][], movingPair = false): boolean {
        const moveWhat = warehouse[from.y][from.x];
        if (moveWhat === "#") {
            return false;
        }
    
        let to: Point;
        switch (direction) {
            case Direction.UP: to = {x: from.x, y: from.y - 1}; break;
            case Direction.DOWN: to = {x: from.x, y: from.y + 1}; break;
            case Direction.LEFT: to = {x: from.x - 1, y: from.y}; break;
            case Direction.RIGHT: to = {x: from.x + 1, y: from.y}; break;
        }
    
        const allowMove = warehouse[to.y][to.x] === "."
            || (direction === Direction.UP && tryMove({x: from.x, y: from.y - 1}, direction, warehouse))
            || (direction === Direction.DOWN && tryMove({x: from.x, y: from.y + 1}, direction, warehouse))
            || (direction === Direction.LEFT && tryMove({x: from.x - 1, y: from.y}, direction, warehouse))
            || (direction === Direction.RIGHT && tryMove({x: from.x + 1, y: from.y}, direction, warehouse));
    
        if (allowMove) {
            // Part 1 logic handles horizontal movement of larger boxes just fine. Needs special handling for vertical movement
            if (!movingPair && (direction === Direction.UP || direction === Direction.DOWN)) {
                if (moveWhat === "[" && !tryMove({x: from.x + 1, y: from.y}, direction, warehouse, true)) {
                    return false;
                }
                if (moveWhat === "]" && !tryMove({x: from.x - 1, y: from.y}, direction, warehouse, true)) {
                    return false;
                }
            }
    
            // Make the move
            warehouse[to.y][to.x] = moveWhat;
            warehouse[from.y][from.x] = ".";
            return true;
        }
        return false;
    }
    
    function findStartLocation(warehouse: string[][]): Point {
        for (let y = 0; y < warehouse.length; y++) {
            for (let x = 0; x < warehouse[y].length; x++) {
                if (warehouse[y][x] === "@") {
                    return {x,y};
                }
            }
        }
    
        throw new Error("Could not find start location!");
    }
    
  •  janAkali   ( @janAkali@lemmy.one ) 
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    222 hours ago

    Nim

    Very fiddly solution with lots of debugging required.

    Code
    type
      Vec2 = tuple[x,y: int]
      Box = array[2, Vec2]
      Dir = enum
        U = "^"
        R = ">"
        D = "v"
        L = "<"
    
    proc convertPart2(grid: seq[string]): seq[string] =
      for y in 0..grid.high:
        result.add ""
        for x in 0..grid[0].high:
          result[^1] &= (
            if grid[y][x] == 'O': "[]"
            elif grid[y][x] == '#': "##"
            else: "..")
    
    proc shiftLeft(grid: var seq[string], col: int, range: HSlice[int,int]) =
      for i in range.a ..< range.b:
        grid[col][i] = grid[col][i+1]
      grid[col][range.b] = '.'
    
    proc shiftRight(grid: var seq[string], col: int, range: HSlice[int,int]) =
      for i in countDown(range.b, range.a+1):
        grid[col][i] = grid[col][i-1]
      grid[col][range.a] = '.'
    
    proc box(pos: Vec2, grid: seq[string]): array[2, Vec2] =
      if grid[pos.y][pos.x] == '[':
        [pos, (pos.x+1, pos.y)]
      else:
        [(pos.x-1, pos.y), pos]
    
    proc step(grid: var seq[string], bot: var Vec2, dir: Dir) =
      var (x, y) = bot
      case dir
      of U:
        while (dec y; grid[y][x] != '#' and grid[y][x] != '.'): discard
        if grid[y][x] == '#': return
        if grid[bot.y-1][bot.x] == 'O': swap(grid[bot.y-1][bot.x], grid[y][x])
        dec bot.y
      of R:
        while (inc x; grid[y][x] != '#' and grid[y][x] != '.'): discard
        if grid[y][x] == '#': return
        if grid[bot.y][bot.x+1] == 'O': swap(grid[bot.y][bot.x+1], grid[y][x])
        inc bot.x
      of L:
        while (dec x; grid[y][x] != '#' and grid[y][x] != '.'): discard
        if grid[y][x] == '#': return
        if grid[bot.y][bot.x-1] == 'O': swap(grid[bot.y][bot.x-1], grid[y][x])
        dec bot.x
      of D:
        while (inc y; grid[y][x] != '#' and grid[y][x] != '.'): discard
        if grid[y][x] == '#': return
        if grid[bot.y+1][bot.x] == 'O': swap(grid[bot.y+1][bot.x], grid[y][x])
        inc bot.y
    
    proc canMoveVert(box: Box, grid: seq[string], boxes: var HashSet[Box], dy: int): bool =
      boxes.incl box
      var left, right = false
      let (lbox, rbox) = (box[0], box[1])
      let lbigBox = box((lbox.x, lbox.y+dy), grid)
      let rbigBox = box((rbox.x, lbox.y+dy), grid)
    
      if grid[lbox.y+dy][lbox.x] == '#' or
         grid[rbox.y+dy][rbox.x] == '#': return false
      elif grid[lbox.y+dy][lbox.x] == '.': left = true
      else:
        left = canMoveVert(box((lbox.x,lbox.y+dy), grid), grid, boxes, dy)
    
      if grid[rbox.y+dy][rbox.x] == '.': right = true
      elif lbigBox == rbigBox: right = left
      else:
        right = canMoveVert(box((rbox.x, rbox.y+dy), grid), grid, boxes, dy)
    
      left and right
    
    proc moveBoxes(grid: var seq[string], boxes: var HashSet[Box], d: Vec2) =
      for box in boxes:
        grid[box[0].y][box[0].x] = '.'
        grid[box[1].y][box[1].x] = '.'
      for box in boxes:
        grid[box[0].y+d.y][box[0].x+d.x] = '['
        grid[box[1].y+d.y][box[1].x+d.x] = ']'
      boxes.clear()
    
    proc step2(grid: var seq[string], bot: var Vec2, dir: Dir) =
      case dir
      of U:
        if grid[bot.y-1][bot.x] == '#': return
        if grid[bot.y-1][bot.x] == '.': dec bot.y
        else:
          var boxes: HashSet[Box]
          if canMoveVert(box((x:bot.x, y:bot.y-1), grid), grid, boxes, -1):
            grid.moveBoxes(boxes, (0, -1))
            dec bot.y
      of R:
        var (x, y) = bot
        while (inc x; grid[y][x] != '#' and grid[y][x] != '.'): discard
        if grid[y][x] == '#': return
        if grid[bot.y][bot.x+1] == '[': grid.shiftRight(bot.y, bot.x+1..x)
        inc bot.x
      of L:
        var (x, y) = bot
        while (dec x; grid[y][x] != '#' and grid[y][x] != '.'): discard
        if grid[y][x] == '#': return
        if grid[bot.y][bot.x-1] == ']': grid.shiftLeft(bot.y, x..bot.x-1)
        dec bot.x
      of D:
        if grid[bot.y+1][bot.x] == '#': return
        if grid[bot.y+1][bot.x] == '.': inc bot.y
        else:
          var boxes: HashSet[Box]
          if canMoveVert(box((x:bot.x, y:bot.y+1), grid), grid, boxes, 1):
            grid.moveBoxes(boxes, (0, 1))
            inc bot.y
    
    
    proc solve(input: string): AOCSolution[int, int] =
      let chunks = input.split("\n\n")
      var grid = chunks[0].splitLines()
      let movements = chunks[1].splitLines().join().join()
    
      var robot: Vec2
      for y in 0..grid.high:
        for x in 0..grid[0].high:
          if grid[y][x] == '@':
            grid[y][x] = '.'
            robot = (x,y)
    
      block p1:
        var grid = grid
        var robot = robot
        for m in movements:
          let dir = parseEnum[Dir]($m)
          step(grid, robot, dir)
        for y in 0..grid.high:
          for x in 0..grid[0].high:
            if grid[y][x] == 'O':
              result.part1 += 100 * y + x
    
      block p2:
        var grid = grid.convertPart2()
        var robot = (robot.x*2, robot.y)
        for m in movements:
          let dir = parseEnum[Dir]($m)
          step2(grid, robot, dir)
          #grid.inspect(robot)
    
        for y in 0..grid.high:
          for x in 0..grid[0].high:
            if grid[y][x] == '[':
              result.part2 += 100 * y + x
    

    Codeberg Repo

  • Haskell

    This was a fun one! I’m quite pleased with moveInto, which could be easily extended to support arbitrary box shapes.

    Solution
    import Control.Monad
    import Data.Bifunctor
    import Data.List
    import Data.Map (Map)
    import Data.Map qualified as Map
    import Data.Set (Set)
    import Data.Set qualified as Set
    
    type C = (Int, Int)
    
    readInput :: String -> (Map C Char, [C])
    readInput s =
      let (room, _ : moves) = break null $ lines s
       in ( Map.fromList [((i, j), c) | (i, l) <- zip [0 ..] room, (j, c) <- zip [0 ..] l],
            map dir $ concat moves
          )
      where
        dir '^' = (-1, 0)
        dir 'v' = (1, 0)
        dir '<' = (0, -1)
        dir '>' = (0, 1)
    
    moveInto :: Int -> Set C -> C -> C -> Set C -> Maybe (Set C)
    moveInto boxWidth walls (di, dj) = go
      where
        go (i, j) boxes
          | (i, j) `Set.member` walls = Nothing
          | Just j' <- find (\j' -> (i, j') `Set.member` boxes) $ map (j -) [0 .. boxWidth - 1] =
              Set.insert (i + di, j' + dj)
                <$> foldM
                  (flip go)
                  (Set.delete (i, j') boxes)
                  [(i + di, j' + z + dj) | z <- [0 .. boxWidth - 1]]
          | otherwise = Just boxes
    
    runMoves :: (Map C Char, [C]) -> Int -> Int
    runMoves (room, moves) scale = score $ snd $ foldl' move (start, boxes) moves
      where
        room' = Map.mapKeysMonotonic (second (* scale)) room
        Just start = fst <$> find ((== '@') . snd) (Map.assocs room')
        walls =
          let ps = Map.keysSet $ Map.filter (== '#') room'
           in Set.unions [Set.mapMonotonic (second (+ z)) ps | z <- [0 .. scale - 1]]
        boxes = Map.keysSet $ Map.filter (== 'O') room'
        move (pos@(i, j), boxes) dir@(di, dj) =
          let pos' = (i + di, j + dj)
           in maybe (pos, boxes) (pos',) $ moveInto scale walls dir pos' boxes
        score = sum . map (\(i, j) -> i * 100 + j) . Set.elems
    
    main = do
      input <- readInput <$> readFile "input15"
      mapM_ (print . runMoves input) [1, 2]